1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a top nozzle incorporating improvements which eliminate relative sliding engagement between the upper support plate of the reactor core and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of a reconstitutable fuel assembly as well as desired alignment of the fuel assembly with the upper core support plate.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 and 3,814,667 to Klumb et al and U.S. Pat. No. 4,269,661 to Kmonk et al.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its usefuel life.
One type of such reconstitutable fuel assembly can be seen in the aforementioned Klumb et al patents. The fuel assembly of Klumb et al includes a top nozzle which incorporates a hold-down plate and also coil springs coaxially disposed about upwardly extending alignment posts. The alignment posts extend through an upper end plate, spaced below the hold-down plate, and are joined thereto and to the upper ends of the guide thimbles with fastener nuts located on the underside of the upper plate. The upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the end plate. A radially enlarged shoulder on the upper end of each of the alignment posts retains the hold-down plate on the posts.
However, the Klumb et al reconstitutable fuel assembly involves a top nozzle arrangement which is difficult to remove and reattach both due to the location of the fasteners and because removal appears to cause the hold-down device of the top nozzle to come apart, requiring added steps and apparatus to prevent this or to later reassembly the hold-down device. The reconstitutable fuel assembly described and illustrated in the patent application cross-referenced above includes design improvements which overcome the aforementioned problems and shortcomings of the Klumb et al top nozzle design. Particularly, the top nozzle of the cross-referenced application is adapted to be removed and then replaced as a unitary subassembly on the guide thimbles.
Notwithstanding these improvements, several additional problems are inherent in the Klumb et al design. Only recognized recently, these problems are not addressed in the Kmonk et al patent nor in the improved top nozzle design of the cross-referenced application and so likewise are present in them. These problems arise from the impingement of coolant cross-flow from adjacent fuel assemblies on the hold-down springs of a given fuel assembly and the relative sliding engagement allowed between the upper ends of the alignment posts (or guide tube extensions) and the upper core support plate at the region of the holes defined therein which receive the upper ends of the alignment posts.
With regard to the first problem, cross-flow from adjacent fuel assemblies occurs because of the radial flow maldistribution across pressurized water reactor cores which is caused by core inlet flow maldistribution and by temperature differences across the core. Thus, there is a radial pressure gradient at the fuel assembly outlet which induces cross-flow above the fuel rods of the assembly. The hold-down springs in the Klumb et al type top nozzle are exposed to the cross-flow which has led to spring failure due to fatigue caused by flow induced vibration.
With regard to the second problem, relative motion occurs between the upper ends of the alignment posts and the upper core support plate because fuel assembly alignment with the upper core plate is accomplished by projecting the guide thimble mounted posts into the holes in the core plate and such alignment must accommodate axially-directed differential growth of the fuel assembly due to differential thermal expansion and irradiation growth. Core plate vibration also produces relative motion between the core plate hole and the thimble alignment post. Such relative motion accompanied by contact between the upper ends of the alignment posts and the hole region of the core plate results in wear of the core plate. Small amplitude vibration, even at low frequencies, can lead to appreciable wear when considered over long periods of time. Since the core plate has a forty year life, wear at the hole region therein can lead to expensive core plate rework to resize the holes.
Consequently, a need exists for a fresh approach to reconstitutable fuel assembly top nozzle design with the objective of eliminating the aforementioned problems of core plate wear and hold-down spring fatigue while retaining the capability of removal and reattachment of the top nozzle without the possibility of its hold-down device coming apart.